This invention generally relates to coin operated devices, and more particularly relates to apparatus and methods used to discriminate coins based on their metal content.
As is well known, there are a variety of coin operated devices such as laundromat equipment, vending machines, toll booths, and public telephones. Generally, such devices identify a deposited coin or token by detecting coin characteristics or parameters, and comparing them to corresponding standards that are known for acceptable coin denominations. For example, some of these parameters are coin diameter, thickness, ferrous or metal content, and weight. Some of the more successful coin discrimination schemes employ a combination of parameters such as coin diameter discrimination combined with sensing the coin's metal characteristics.
There are a variety of prior art devices for measuring the metal content of coins. For example, in one arrangement the coin passes over an inductor which is part of a Colpitts oscillator circuit, and the metal in the coin alters the inductance of the circuit. In particular, when a ferromagnetic coin shunts an inductor in an alternating current circuit, the direction of change in inductance of the inductor depends on both the skin effect and the effect of shunting by the magnetic material. Generally, if the magnetic permeability of a coin is high and the conductivity is low, the inductance of the inductor will increase. If the permeability is low and the conductivity is high, the inductance will decrease. The metallic or ferrous content is then derived by measuring the amplitude of the oscillator output signal, and comparing it with known references.
One prior art metal content discriminating apparatus is described in U.S. Pat. No. 3,870,137. A coin is subjected to electromagnetic fields of at least two substantially different frequencies. A determination is made for each of the examination frequencies whether or not the interaction between the coin being tested and the electromagnetic field of that frequency produces the interaction effect within predetermined tolerances which is anticipated for acceptable electrically conductive coins.
Another prior art method and apparatus for metal content discrimination is described in U.S. Pat. No. 3,966,304. The disclosed method includes the steps of generating an alternating magnetic field, placing the coin to be tested with one face toward the source of the field, and comparing the phases of the field adjacent the two faces of the coin. This is practically accomplished by passing a first AC signal through a first wire thereby inducing a second AC signal in a second wire spaced from the first wire, placing a coin between the wires so as to shield direct paths from one wire to the other, and comparing the phases of the two AC signals. In a variation of this method, a third wire on the same side of the coin as the first wire may be used to sense the phase of the field on that side on the coin, in which case the phase of the AC signal induced in the third wire would be compared with that of the second AC signal.
One characteristic of these prior art metal content discrimination devices is that the coin rolls by the metal sensor or coil. In particular, the coin generally drops onto a coin track between sidewalls and rolls down the coin track on its edge under the influence of gravity. The sidewalls are parallel plates spaced apart by at least slightly more than the thickness of the thickest coin to be processed by the device. In addition, the sidewalls are typically tilted from the vertical so that a face on a coin rolling down the coin track bears on the metal content sensor or coil. Whether the measured parameter is amplitude, frequency, or phase, motion of a coin leads to inconsistencies that result in inaccurate measurements.
Another prior art metal sensor is described in U.S. Pat. No. 4,936,436 wherein the coin remains in the user's hand until it is validated as a proper coin. In particular, the coin goes in approximately one third of the way at which point the leading edge is detected. At this point, while the user still has a hold of the coin, the inserted portion is between two coils. An evaluation is done to determine if the coin is acceptable. If it is, the coin passage is opened, and the coin is received. With this method, the spacial disposition of the coin with respect to the coils may vary from use to use thereby detracting form the precision and repeatability of measurements.